rpi_pico: Fix compilation for non Linux systems
rpi_pico: Fix compilation for non Linux systems

On non Linux system, the compilation failed because the main
CMake did not pass the CMAKE_SYSTEM_NAME to the bootloader's
CMake. This caused the variable to be empty, instead of being
"Generic".

Signed-off-by: Yonatan Schachter <c03ea33f3e5919b7d64cfd097e68ce03a1b1a328@gmail.com>

finikorg/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr